The importance of monitoring urine output from a critically ill patient is well known. Diminished urine output is an early sign of changing patient conditions, for example, renal failure, liver failure, or congestive heart failure. Acute changes in urine output can indicate deteriorating patient conditions even before changes in patient vital signs of blood pressure, temperature, pulse, or respiration.
In patients who are suffering renal insufficiency, bedside hemofiltration, such as continuous veno-venous hemofiltration, continuous arteriovenous hemofiltration, or peritoneal dialysis, may be required to remove ultrafiltrate (i.e. the plasma/water content of the blood). Balancing fluid input against ultrafiltrate output is crucial to maintaining proper fluid balance for the patient. Thus the importance of continuously monitoring ultrafiltrate output is well known.
Not only is monitoring the volume of fluid output crucial in the case of hemofiltration, but monitoring the rate of fluid output is equally critical. Changes in the flow rate of the ultrafiltrate output are predictive of complications such as excessive clotting of blood stopping up the hemofilter. Early warning of such clotting, as indicated by a decrease in the flow rate of the ultrafiltrate output, gives the physician the opportunity to inject heparin into the system proximal to the hemofilter to arrest clotting.
Electronic meters for continuously monitoring the fluid output of a patient are well known. Examples of typical prior art urine monitors are shown in U.S. Pat. No. 4,448,207 and U.S. Pat. No. 4,658,834. A disposable container is inserted into a reusable housing having an electronic module. Patient fluid output is discharged into the container. An ultrasound transducer is mounted to the housing adjacent the container and is acoustically coupled to a wall of the container. When the sound wave hits the interface between the air and the liquid in the container, the signal is reflected. The return signal from the ultrasound transducer is input into the electronic module, which continuously monitors the contents of the container for volume changes. In addition the electronic module continuously monitors a sensor located in a Foley catheter for changes in temperature. The device displays patient information such as volume and rate of patient fluid output on a display panel located at the upper end of the housing.
Because electronic monitors of the types disclosed in the aforementioned U.S. Pat. No. 4,448,207 and U.S. Pat. No. 4,658,834 rely on reflection of sound waves off of the air/liquid interface, these monitors encounter difficulties if the monitor is tilted from vertical. If the monitor is tilted, the air/liquid interface is no longer at a redefined angle relative to the axis of propagation of the sound waves, and the reflected signal returns at an undesirable angle. Consequently the signal received by the transducer is weak, sometimes too weak to register. This problem is exacerbated if a foreign substance is present, e.g. blood in the urine, as such substance will further attenuate the return signal.
Thus there is a need for an electronic fluid output monitor which provides accurate readings even when the monitor is tilted from vertical.
There is a further need for an electronic fluid output monitor which provides accurate readings even when a foreign substance such as blood exists in the patient's fluid output.
Another problem inherent in fluid output monitors of the type disclosed in the aforementioned U.S. Pat. No. 4,448,207 and U.S. Pat. No. 4,658,834 is that such monitors are incapable of accurately measuring small volumes of liquid. Because the depth of the liquid in the container is extremely shallow as patient fluid output is first introduced into the container, and because the liquid may not initially uniformly fill the container as a result of the effects of surface tension, prior art electronic fluid output monitors usually cannot measure volumes of less than 5 cc of fluid.
Thus there is a need for an electronic fluid output monitor which permits measurement of small volumes of liquid.